Added generic asynchronous operations code.

Removed data_in_len and data_out_len from ata_command structure; the
lengths are implied by the sector count and the presence of the data_in
or data_out pointers.

Changed AoE code to use subcommands by default, and made aoe_issue()
nonblocking (with completion via async_wait()).

src/include/gpxe/async.h

@@ -0,0 +1,62 @@
+#ifndef _GPXE_ASYNC_H
+#define _GPXE_ASYNC_H
+
+/** @file
+ *
+ * Asynchronous operations
+ *
+ */
+
+#include <errno.h>
+#include <assert.h>
+
+/** An asynchronous operation */
+struct async_operation {
+	/** Operation status
+	 *
+	 * This is an error code as defined in errno.h, plus an offset
+	 * of EINPROGRESS.  This means that a status value of 0
+	 * corresponds to a return status code of -EINPROGRESS,
+	 * i.e. that the default state of an asynchronous operation is
+	 * "not yet completed".
+	 */
+	int status;
+};
+
+/**
+ * Set asynchronous operation status
+ *
+ * @v aop	Asynchronous operation
+ * @v rc	Return status code
+ */
+static inline __attribute__ (( always_inline )) void
+async_set_status ( struct async_operation *aop, int rc ) {
+	aop->status = ( rc + EINPROGRESS );
+}
+
+/**
+ * Get asynchronous operation status
+ *
+ * @v aop	Asynchronous operation
+ * @ret rc	Return status code
+ */
+static inline __attribute__ (( always_inline )) int
+async_status ( struct async_operation *aop ) {
+	return ( aop->status - EINPROGRESS );
+}
+
+/**
+ * Flag asynchronous operation as complete
+ *
+ * @v aop	Asynchronous operation
+ * @v rc	Return status code
+ */
+static inline __attribute__ (( always_inline )) void
+async_done ( struct async_operation *aop, int rc ) {
+	assert ( rc != -EINPROGRESS );
+	async_set_status ( aop, rc );
+}
+
+extern int async_wait ( struct async_operation *aop );
+
+#endif /* _GPXE_ASYNC_H */